# Re: Calculating Capacitance of solid disk

```Original poster: "Barton B. Anderson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <classictesla-at-netzero-dot-com>

Dan, Jon, All -

Jon, if this test is similar to one I've done before where we find the
differential of two frequency's, I seem to recall that getting a good
signal was tough due to the simple scope I was using. I think if Dan has a
decent scope, then it would be great to see him perform this test (even if
just an academic exercise).

But, as you mentioned, should the disc be placed on the coil, the effective
capacitance of the disc will be less than the isotropic capacitance. It
should be easy enough to take the coil outside away from walls (eliminate
external capacitance's) and measure the secondary frequency by feeding a
signal to the base and dangling a scope probe with a small wire as the
antenna several feet from the coil. A couple sheets of aluminum foil serve
well as the ground plane. This will give the true Cself of the coil. Then
install the disk and take the measurement again. The effective capacitance
of the disk can be calculated from those two frequency's and requires only
a simple scope and generator.

Take care,
Bart

Tesla list wrote:

>Original poster: "John H. Couture by way of Terry Fritz
><teslalist-at-qwest-dot-net>" <couturejh-at-mgte-dot-com>
>
>
>Bart, Dan, All -
>
>One advantage of this new disc is that the capacitance can be easily
>measured to check with what computer programs and estimates are giving.
>There can't be many disc capacitance test that have ever been made so these
>would be important tests.
>
>One recommended test is the one that is shown in the "Tesla Coil
>Construction Guide" page 14-13. All that is needed to find are two test
>frequencies. The test should be made with a small TC with an operating
>frequency of about 600 to 800 Khz. The frequency differencies will be small.
>
>One capacitance equation for a disc from the TCBA News is
>
>    pf = .35 d   where d = centimeters
>                 and  pf = isotropic capacity
>
>My dictionary defines "isotropic" as "exibiting properties (as velocity of
>light transmission) with the same values when measured along axes in all
>directions". This has confused coilers for years.
>
>So we now have -
>           Fantc       3.56   pf
>           TCBA        3.556  pf
>           Estimated   3.98   pf
>
>It should be noted that when a toroid is placed on a TC secondary the toroid
>capacitance is reduced because of the proximity of the secondary. However,
>the toroid isotropic capacity to earth increases as it is brought closer to
>earth. This means that the test resonant frequency will be LESS than
>expected compared to the resonant frequency when the secondary has no
>terminal. Don't let these resonant frequencies confuse the testing. If the
>disc behaves like the toroid the testing should show that the disc capacity
>is greater than what is shown above?
>
>John Couture
>
>-------------------------------
>
>
>-----Original Message-----
>From: Tesla list [mailto:tesla-at-pupman-dot-com]
>Sent: Sunday, March 09, 2003 11:11 PM
>To: tesla-at-pupman-dot-com
>Subject: Re: Calculating Capacitance of solid disk
>
>
>Original poster: "Barton B. Anderson by way of Terry Fritz
><teslalist-at-qwest-dot-net>" <classictesla-at-netzero-dot-com>
>
>Steve,
>
>You are correctly, I messed up the inputs. Yes, I put in meters instead of
>inches. Thanks for paying attention and letting me know.
>
>Now, with the "correct" inputs, computations for the 4" disc capacitance is:
>
>System or Object Capacitance: 3.56 pF
>Run CompleteTime elapsed: 2.1 secs
>
>Take care,
>Bart
>
>Tesla list wrote:
>
> >Original poster: "Stephen Conner by way of Terry Fritz
> ><teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>
> >
> >At 11:31 09/03/03 -0700, you wrote:
> >
> >>Original poster: "Barton B. Anderson by way of Terry Fritz
> >><teslalist-at-qwest-dot-net>" <classictesla-at-netzero-dot-com>
> >>
> >>Hi Dan,
> >>
> >>This is a nice function of Fantc. It can perform analysis of object
> >>capacitance for toroids, spheres, spheroids, discs, and cylinders (up to
> >>10 each of wanted). In this case, I simply computed the 4" disk
> >>capacitance. Here's the output at a detail 3 setting.
> >>
> >>Computing ..... please wait
> >>System or Object Capacitance: 140.08 pF
> >>Run CompleteTime elapsed: 2.1 secs
> >
> >
> >That's way too high! A disk that size should be way less than 10pF. are
> >you sure you didn't get inches and meters mixed up?
> >
> >Steve C.
>
>
>
>

```